Neoproterozoic Carbon Cycle and the Rise of Oxygen

Paleoenvironmental reconstructions rely heavily on geochemical proxy records from marine sediments.  Along with dramatic changes in climate and biology, d13Cstudies through Neoproterozoic strata host some of the largest perturbations to the carbon cycle in the geological record [e.g. 10,16].  Neoproterozoic negative d13Cexcursions have been recently attributed to the remineralization of a giant dissolved organic carbon (DOC) pool, which would have entailed the consumption of massive volumes of oxidants [e.g. 29,31].  The large DOC model was based on the lack of covariance in organic carbon isotopes (d13Corg) and carbonate carbon isotopes (d13Ccarb) from carbonate samples with very low organic carbon content (TOC).  In Mongolia, we discovered a new, large d13Ccarb excursion (from +8‰ to -7‰ back to +10‰ over ~50 m of strata) in relatively TOC-rich carbonate, referred to as the Tayshir excursion [10].  With colleagues at Harvard, we then demonstrated that d13Corg and d13Ccarb covaried through the Tayshir excursion, and argued that a large DOC pool never existed, but rather, the apparent lack of covariance in previous studies was due to masking of detrital and migrated organic matter in TOC-poor rocks [29].  These new data have opened up an alternative framework to reinterpret the d13Crecord through Earth history that is not in direct conflict with oxidant budgets [31].

The appearance of macroscopic fossils in the Neoproterozoic has long been assumed to be related to a rise in atmospheric oxygen, however, independent evidence for an oxygenation event has remained elusive [e.g. 35].  Previous studies in NW Canada correlated the first appearance datum (FAD) of Ediacaran fauna with a shift in Fe speciation data that was interpreted to represent an oxygenation of Ediacaran oceans [L].  Our recent mapping has demonstrated that the FAD of Ediacaran fauna occur above a major sequence boundary, and are not associated with the change in Fe speciation data. We also discovered a large negative d13C excursion through these strata that we correlate to the Shuram excursion [M]. We further proposed that the Shuram anomaly was driven by changes in the size and location of the global authigenic carbonate reservoir [31,36]. This anomaly occurs above the FAD of Ediacaran fauna, and directly below the FAD of bioturbation. We are currently exploiting unique late Neoproterozoic to Early Cambrian records in NW Canada and Mongolia to further elucidate the relationships between the appearance of large animals, the evolution of mobility, mixing of the sediment-water interface, changes in the carbon cycle, and the putative rise in atmospheric oxygen. Preliminary paired d13Ccarb and d13Corg results through the Early Cambrian d13Ccarb oscillations in TOC-rich limestone in Mongolia display large lateral gradients and lack covariance, both indicative of authigenesis [consistent with 31].  Coupled with the fact that these strata host phosphorites and some of the richest trace fossil and small-shelly fossil records in the world, our studies in Mongolia promise to enrich our understanding of the coevolution of life and the environment for many years to come.